The rated current of the brushless DC motor is generally required by the user, and it involves the power and discharge capacity (ampere-hour) of the DC power source selected by the user. The smaller the current drawn by the motor from the battery, the longer the battery can be used on a single charge. Especially in electric bicycle motors, users want the length of the motor to be longer. The rated current of the motor is relatively small and the motor is rated. The determination of the current is very particular. Therefore, the factors affecting the rated current of the brushless DC motor are as follows.
(1) The input power of the motor at the rated point is determined, and the efficiency of the motor is basically determined. The higher the rated voltage of the motor, the lower the rated current.
(2) The larger the torque constant of the motor, the smaller the rated current of the motor.
(3) Minimize the torque of the motor, that is, reduce the no-load loss of the motor, and reduce the no-load current of the motor. In this way, the rated current of the motor will be reduced accordingly, but the reduction is not much.
After the rated voltage of the brushless DC motor is increased, the no-load speed of the motor will also increase. If the rated speed of the motor is determined, the efficiency of the motor is not much different, then the no-load speed of the motor can be calculated. If the rated voltage of the motor is increased, in order to ensure the rated speed of the motor, it means that the back electromotive force constant of the motor should be increased in proportion to the rated voltage. If the shape of the motor is unchanged, that is, the working magnetic flux of the motor is unchanged, then the number of effective conductors of the motor must be increased proportionally. For a certain area of the groove, this is not a problem, and the armature resistance of the motor will be corresponding. Increase, the internal resistance of the motor is large.
But the current of the motor also decreases accordingly. In the copper loss of the motor, the decrease of the current has a square relationship, while the increase of the resistance of the motor has a proportional relationship. Therefore, the copper loss of the motor will not increase, but will decrease. Therefore, the efficiency of the same motor as a high-voltage motor will be higher than that of a low-voltage motor, mainly because the copper loss of the motor is smaller. Power transmission uses high-voltage power transmission, which is to reduce the loss caused by the current in the line. The high voltage will cause damage to the human body, so the correct selection of the power supply voltage of the DC brushless motor is the designer needs to seriously consider.
